Reductive elimination/oxidative addition of carbon-hydrogen bonds at Pt(IV)/Pt(II) centers: mechanistic studies of the solution thermolyses of Tp(Me2)Pt(CH3)2H

Reductive elimination of methane occurs upon solution thermolysis of kappa(3)-Tp(Me)2Pt(IV)(CH(3))(2)H (1, Tp(Me)2 = hydridotris(3,5-dimethylpyrazolyl)borate). The platinum product of this reaction is determined by the solvent. C-D bond activation occurs after methane elimination in benzene-d(6), to yield kappa(3)-Tp(Me)2Pt(IV)(CH(3))(C(6)D(5))D (2-d(6)), which undergoes a second reductive elimination/oxidative addition reaction to yield isotopically labeled methane and kappa(3)-Tp(Me)2Pt(IV)(C(6)D(5))(2)D (3-d(11)). In contrast, kappa(2)-Tp(Me)2Pt(II)(CH(3))(NCCD(3)) (4) was obtained in the presence of acetonitrile-d(3), after elimination of methane from 1. Reductive elimination of methane from these Pt(IV) complexes follows first-order kinetics, and the observed reaction rates are nearly independent of solvent. Virtually identical activation parameters (DeltaH(++)(obs) = 35.0 +/- 1.1 kcal/mol, DeltaS(++)(obs) = 13 +/- 3 eu) were measured for the reductive elimination of methane from 1 in both benzene-d(6) and toluene-d(8). A lower energy process (DeltaH(++)(scr) = 26 +/- 1 kcal/mol, DeltaS(++)(scr) = 1 +/- 4 eu) scrambles hydrogen atoms of 1 between the methyl and hydride positions, as confirmed by monitoring the equilibration of kappa(3)-Tp(Me)()2Pt(IV)(CH(3))(2)D (1-d(1)()) with its scrambled isotopomer, kappa(3)-Tp(Me)2Pt(IV)(CH(3))(CH(2)D)H (1-d(1'). The sigma-methane complex kappa(2)-Tp(Me)2Pt(II)(CH(3))(CH(4)) is proposed as a common intermediate in both the scrambling and reductive elimination processes. Kinetic results are consistent with rate-determining dissociative loss of methane from this intermediate to produce the coordinatively unsaturated intermediate [Tp(Me)2Pt(II)(CH(3))], which reacts rapidly with solvent. The difference in activation enthalpies for the H/D scrambling and C-H reductive elimination provides a lower limit for the binding enthalpy of methane to [Tp(Me)2Pt(II)(CH(3))] of 9 +/- 2 kcal/mol.     

Nickel-catalyzed addition of benzenethiol to alkynes: Formation of carbon-sulfur and carbon-carbon bonds

Nickel-catalyzed addition of benzenethiol to alkynes leads to alkenyl and dienyl sulfides; the direction of the process can be controlled by varying the PhSH/alkyne ratio. An advanced procedure, which ensures higher yields of 2-phenylsulfanylalkenes, includes gradual addition of alkyne to the other reactants. The structures of conjugated dienyl sulfides formed in the reaction were determined by 2D NMR spectroscopy. Dedicated to Academician O. M. Nefedov on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2030–2034, November, 2006.     

Reactivity of Pt0 complexes toward gallium(III) halides: synthesis of a platinum gallane complex and oxidative addition of gallium halides to Pt0

The reaction of [Pt(PCy 3) 2] and GaCl 3 resulted in quantitative formation of the adduct [(Cy 3P) 2Pt-GaCl 3], the first known platinum gallane complex. Although similar reactivity with GaBr 3 and GaI 3 was expected, NMR spectroscopic data revealed a different reaction course. Crystal structure determination proved that, in the latter case, the product of the oxidative addition was formed. The resulting platinum gallyl complexes represent the first example of an oxidative addition of gallium(III) halides to low-valent transition metals.     

Competing C-F activation pathways in the reaction of Pt(0) with fluoropyridines: phosphine-assistance versus oxidative addition

A survey of computed mechanisms for C-F bond activation at the 4-position of pentafluoropyridine by the model zero-valent bis-phosphine complex, [Pt(PH3)(PH2Me)], reveals three quite distinct pathways leading to square-planar Pt(II) products. Direct oxidative addition leads to cis-[Pt(F)(4-C5NF4)(PH3)(PH2Me)] via a conventional 3-center transition state. This process competes with two different phosphine-assisted mechanisms in which C-F activation involves fluorine transfer to a phosphorus center via novel 4-center transition states. The more accessible of the two phosphine-assisted processes involves concerted transfer of an alkyl group from phosphorus to the metal to give a platinum(alkyl)(fluorophosphine), trans-[Pt(Me)(4-C5NF4)(PH3)(PH2F)], analogues of which have been observed experimentally. The second phosphine-assisted pathway sees fluorine transfer to one of the phosphine ligands with formation of a metastable metallophosphorane intermediate from which either alkyl or fluorine transfer to the metal is possible. Both Pt-fluoride and Pt(alkyl)(fluorophosphine) products are therefore accessible via this route. Our calculations highlight the central role of metallophosphorane species, either as intermediates or transition states, in aromatic C-F bond activation. In addition, the similar computed barriers for all three processes suggest that Pt-fluoride species should be accessible. This is confirmed experimentally by the reaction of [Pt(PR3)2] species (R = isopropyl (iPr), cyclohexyl (Cy), and cyclopentyl (Cyp)) with 2,3,5-trifluoro-4-(trifluoromethyl)pyridine to give cis-[Pt(F){2-C5NHF2(CF3)}(PR3)2]. These species subsequently convert to the trans-isomers, either thermally or photochemically. The crystal structure of cis-[Pt(F){2-C5NHF2(CF3)}(P iPr3)2] shows planar coordination at Pt with r(F-Pt) = 2.029(3) A and P(1)-Pt-P(2) = 109.10(3) degrees. The crystal structure of trans-[Pt(F){2-C5NHF2(CF3)}(PCyp3)2] shows standard square-planar coordination at Pt with r(F-Pt) = 2.040(19) A.     

Solvent coordination to palladium can invert the selectivity of oxidative addition

Reaction solvent was previously shown to influence the selectivity of Pd/P^tBu₃-catalyzed Suzuki–Miyaura cross-couplings of chloroaryl triflates. The role of solvents has been hypothesized to relate to their polarity, whereby polar solvents stabilize anionic transition states involving [Pd(P^tBu₃)(X)]⁻ (X = anionic ligand) and nonpolar solvents do not. However, here we report detailed studies that reveal a more complicated mechanistic picture. In particular, these results suggest that the selectivity change observed in certain solvents is primarily due to solvent coordination to palladium. Polar coordinating and polar noncoordinating solvents lead to dramatically different selectivity. In coordinating solvents, preferential reaction at triflate is likely catalyzed by Pd(P^tBu₃)(solv), whereas noncoordinating solvents lead to reaction at chloride through monoligated Pd(P^tBu₃). The role of solvent coordination is supported by stoichiometric oxidative addition experiments, density functional theory (DFT) calculations, and catalytic cross-coupling studies. Additional results suggest that anionic [Pd(P^tBu₃)(X)]⁻ is also relevant to triflate selectivity in certain scenarios, particularly when halide anions are available in high concentrations.

In the presence of the bulky monophosphine P^tBu₃, palladium usually prefers to react with Ar–Cl over Ar–OTf bonds. However, strongly coordinating solvents can bind to palladium, inducing a reversal of selectivity.

Oxidative addition is a key elementary step in diverse transformations catalyzed by transition metals.¹ For instance, this step is common to traditional cross-coupling reactions, which are among the most widely used methods for small molecule synthesis. During the oxidative addition step of cross-coupling reactions, a low valent metal [usually Pd(0)] inserts into a C–X bond with concomitant oxidation of the metal by two electrons. The “X” group of the C–X bond is commonly a halogen or triflate. Despite a wealth of research into this step,^2–5 uncertainties remain about its mechanistic nuances. The mechanistic details are especially pertinent to issues of selectivity that arise when substrates contain more than one potentially reactive C–X bond.⁶One of the best-studied examples of divergent selectivity at the oxidative addition step is the case of Pd-catalyzed Suzuki couplings of chloroaryl triflates. In 2000, Fu reported that a combination of Pd(0) and P^tBu₃ in tetrahydrofuran (THF) effects selective coupling of 1 with o-tolylB(OH)₂via C–Cl cleavage, resulting in retention of the triflate substituent in the final product 2a (Scheme 1A).⁷ In contrast, the use of PCy₃ (ref. 7) or most other phosphines⁸ provides complementary selectivity (product 2b) under similar conditions. The unique selectivity imparted by P^tBu₃ was later attributed to this ligand''s ability to promote a monoligated oxidative addition transition state on account of its bulkiness.^5,8 Smaller ligands, on the other hand, favor bisligated palladium, which prefers to react at triflate. The relationship between palladium''s ligation state and chemoselectivity has been rationalized by Schoenebeck and Houk through a distortion/interaction analysis.⁵ In brief, the selectivity preference of PdL₂ is dominated by a strong interaction between the electron-rich Pd and the more electrophilic site (C–OTf). On the other hand, PdL is less electron-rich and its selectivity preference mainly relates to minimizing unfavorable distortion energy by reacting at the more easily-distorted C–Cl bond.Open in a separate windowScheme 1Seminal reports on the effects of (A) ligands and (B) solvents on the selectivity of cross-coupling of a chloroaryl triflate.^5,7,9Proutiere and Schoenebeck later discovered that replacing THF with dimethylformamide (DMF, Scheme 1B, entry 1) or acetonitrile caused a change in selectivity for the Pd/P^tBu₃ system.^9,10 In these two polar solvents, preferential reaction at triflate was observed, and P^tBu₃ no longer displayed its unique chloride selectivity. The possibility of solvent coordination to Pd was considered, as bisligated Pd(P^tBu₃)(solv) would be expected to favor reaction at triflate. However, solvent coordination was ruled out on the basis of two intriguing studies. First, DFT calculations using the functional B3LYP suggested that solvent-coordinated transition states are prohibitively high in free energy (about 16 kcal mol⁻¹ higher than the lowest-energy monoligated transition structure). Second, the same solvent effect was not observed in a Pd/P^tBu₃-catalyzed base-free Stille coupling in DMF (Scheme 1B, entry 2). Instead, the Stille coupling was reported to favor reaction at chloride despite the use of a polar solvent. This result appears inconsistent with the possibility that solvent coordination induces triflate-selectivity, as coordination of DMF to Pd should be possible in both the Stille and Suzuki conditions, if it happens at all. Instead, it was proposed that the key difference between the Suzuki and Stille conditions was the absence of coordinating anions in the latter (unlike traditional Suzuki couplings, Stille couplings do not necessarily require basic additives such as KF to promote transmetalation). Indeed, when KF or CsF was added to the Stille reaction in DMF, selectivity shifted to favor reaction at triflate (Scheme 1B, entry 3), thereby displaying the same behavior as the Suzuki coupling in this solvent. On the basis of this and the DFT studies, it was proposed that polar solvents induce a switch in chemoselectivity if coordinating anions like fluoride are available by stabilizing anionic bisligated transition structures (Scheme 1B, right).However, our recent extended solvent effect studies produced confounding results.¹¹ In a Pd/P^tBu₃-catalyzed Suzuki cross-coupling of chloroaryl triflate 1, we observed no correlation between solvent polarity and chemoselectivity (Scheme 2). Although some polar solvents such as MeCN, DMF, and dimethylsulfoxide (DMSO) favor reaction at triflate, a number of other polar solvents provide the same results as nonpolar solvents by favoring reaction at chloride. For example, cross-coupling primarily takes place through C–Cl cleavage when the reaction is conducted in highly polar solvents like methanol, water, acetone, and propylene carbonate. In fact, the only solvents that promote reaction at triflate are ones that are commonly thought of as “coordinating” in the context of late transition metal chemistry.¹² These are solvents containing nitrogen, sulfur, or electron-rich oxygen lone pairs (nitriles, DMSO, and amides). The observed solvent effects were upheld for a variety of chloroaryl triflates and aryl boronic acids.¹¹Open in a separate windowScheme 2Expanded solvent effect studies in the Pd/P^tBu₃-catalyzed Suzuki coupling.¹¹We have sought to reconcile these observations with the earlier evidence⁹ against solvent coordination. Herein we report detailed mechanistic studies indicating that coordinating solvents alone are sufficient to induce the observed selectivity switch. In solvents like DMF and MeCN, stoichiometric oxidative addition is favored at C–OTf even in the absence of anionic additives. The apparent contradiction between our observations and the previously-reported DFT calculations and base-free Stille couplings is reconciled by a reevaluation of those studies. In particular, when dispersion is considered in DFT calculations, neutral solvent-coordinated transition structures involving Pd(P^tBu₃)(solv) become energetically feasible. Furthermore, we find that the selectivity analysis in the Stille couplings is convoluted by low yields, the formation of side products, and temperature effects. When these factors are disentangled, the Stille coupling in DMF displays selectivity similar to the Suzuki coupling in the same coordinating solvent. In light of these new results, anionic bisligated [Pd(P^tBu₃)(X)]⁻ does not appear to be the dominant active catalyst in nonpolar or polar solvents unless special measures are taken to increase the concentration of free halide, such as adding tetraalkylammonium halide salts or crown ethers.     

Photochemisch induzierte oxidative addition von aldehyden an dekacarbonyldirhenium(0)

Decacarbonyldirhenium(O) (I) reacts photochemically with acetaldehyde or propionaldehyde (II, III) to give predominantly tetradecacarbonyl-μ-hydridotrirhenium (VI). In addition, μ-acyloctacarbonyl-μ-hydridodirhenium complexes (IV, V) are obtained in oxidative addition reactions with simultaneous loss of two CO ligands. Complexes IV and V were characterized by their elemental analyses, IR, ¹H NMR, and ¹³C NMR spectra. Furthermore, the molecular structure of IV was determined by X-ray structure analysis. IV shows approximate (C_s-symmetry. Two Re(CO₄) groups are connected via the C and O atoms of the carbonyl function of an acetyl bridge, and from the steric arrangement of the Re(CO)₄ moieties and NMR spectroscopic evidence, a hydrido bridge has to be assumed between the Re atoms, which rules out a ReRe bond. The two coordination octahedrons are joined by the hydrido bridge. The connection of two further edges by the acetyl bridge causes an eclipsed arrangement of the CO ligands in IV.     

Mechanism of oxidative addition of benzonitriles to DI(1,4-bis(diethylphosphino)butane)nickel(0)

A kinetic study of the oxidative addition of RC₆H₄CN (R = H, m-CN, p-CN) to Ni(DEPB)₂ (DEPB = 1,4-bis(diethylphosphino)butane) suggests a template mechanism leading to the fission of one C---CN bond. The reaction products are trans-planar cyano-organonickel(II) complexes, Ni₂(μ-DEPB)₂(RC₆H₄)₂(CN)₂ and Ni(η¹- DEPB)(RC₆H₄)(CN), in equilibrium. through exchange of DEPB.     

Accurate modelling of Pd(0) + PhX oxidative addition kinetics

We have used dispersion-corrected DFT (DFT-D) together with solvation to examine possible mechanisms for reaction of PhX (X = Cl, Br, I) with Pd(P(t)Bu(3))(2) and compare our results to recently published kinetic data (F. Barrios-Landeros, B. P. Carrow and J. F. Hartwig, J. Am. Chem. Soc., 2009, 131, 8141-8154). The calculated activation free energies agree near-quantitatively with experimentally observed rate constants.     

Synthesis of complexes bearing NH,NMe-substituted NHCs by oxidative addition of 2-halogenato-N-methylbenzimidazoles to Ni(0)

Reaction of 2-X-N-methylbenzimidazole (X = chloro, iodo) with Ni(0) complexes in the presence of dppe or PEt(3) and an external proton source yielded via an oxidative addition reaction nickel(II) complexes bearing NH,NMe-functionalized NHC ligands.     

The mechanism of oxidative addition of Pd(0) to Si–H bonds: electronic effects,reaction mechanism,and hydrosilylation

The oxidative addition of Pd to Si–H bonds is a crucial step in a variety of catalytic applications, and many aspects of this reaction are poorly understood. One important yet underexplored aspect is the electronic effect of silane substituents on reactivity. Herein we describe a systematic investigation of the formation of silyl palladium hydride complexes as a function of silane identity, focusing on electronic influence of the silanes. Using [(μ-dcpe)Pd]₂ (dcpe = dicyclohexyl(phosphino)ethane) and tertiary silanes, data show that equilibrium strongly favours products formed from electron-deficient silanes, and is fully dynamic with respect to both temperature and product distribution. A notable kinetic isotope effect (KIE) of 1.21 is observed with H/DSiPhMe₂ at 233 K, and the reaction is shown to be 0.5^th order in [(μ-dcpe)Pd]₂ and 1^st order in silane. Formed complexes exhibit temperature-dependent intramolecular H/Si ligand exchange on the NMR timescale, allowing determination of the energetic barrier to reversible oxidative addition. Taken together, these results give unique insight into the individual steps of oxidative addition and suggest the initial formation of a σ-complex intermediate to be rate-limiting. The insight gained from these mechanistic studies was applied to hydrosilylation of alkynes, which shows parallel trends in the effect of the silanes'' substituents. Importantly, this work highlights the relevance of in-depth mechanistic studies of fundamental steps to catalysis.

Mechanistic studies reveal the rate law, an H/D KIE, and that the silane’s electronics impact the thermodynamic and kinetic energetics of the oxidative addition reaction. These electronic effects are relevant in the hydrosilylation of alkynes.     

Heterolytic addition of E-H bonds across Pt-P bonds in Pt N-heterocyclic phosphenium/phosphido complexes

The reactivity of E-H bonds (E = S, O, Cl) with Pt(II) complexes ligated by an N-heterocyclic phosphido-containing diphosphine ligand have been investigated. Addition of PhSH to [(PPP)Pt(PPh(3))][PF(6)] (1) results in clean formation of [(PP(H)P)Pt(SPh)][PF(6)] (3), in which the substrate has added across the Pt-P(NHP) bond. Similar reactivity occurs when 1 is treated with ROH (R = Ph, Me), but in this case the O-H bond adds across the Pt-P bond in the opposite direction producing [(PP(OR)P)Pt(H)(PPh(3))][PF(6)] (R = Ph (4), Me (5)). HCl addition to 1 cleanly generates [(PP(H)P)PtCl][PF(6)] (6(PF6)). The neutral Pt-NHP complex (PPP)PtCl (2) exhibits similar reactivity; however, in the presence of the nucleophilic Cl(-) anion, the (PP(OR)P)Pt(H)Cl species presumably generated via addition of ROH (R = Me, Et) undergoes an Arbuzov-like dealkylation reaction to exclusively form the N-heterocylic phosphinito species (PP(O)P)Pt(H) (7).     

Oxidative addition of Pd0 to Ar-SO2R bonds: heck-type reactions of sulfones

[reaction: see text] Phenyl vinyl sulfones and sulfoxides react with Pd(OAc)(2) to form styryl sulfoxides and sulfones according to the first Mizoroki-Heck reaction reported for these thio derivatives. Only sulfones are able to react by using catalytic amounts of Pd (up to 1 mol %) in the presence of Ag(2)CO(3). 1,2-Diphenylsulfonyl ethenes, alkynylphenyl sulfones, and other sulfones, less prone to act as acceptors in the Heck-type reactions, can transfer the aryl group to alkyl acrylates forming cinnamic esters.     

Lewis acid-promoted oxidative addition of thioimidates to Pd0

The isomeric S-methyldihydropyrrins 9-Z and 9-E exhibit markedly different behavior in Pd(0)-catalyzed cross-coupling reactions. Thioimidates 9-Z are readily converted to imines 10-Z employing Pd(0)/AlkZnI. Under identical conditions 9-E are inert. Oxidative addition to Pd(0) requires activation by Zn or other Lewis acids, which is sterically unfavorable with 9-E. Analogous results were obtained with the related thioimidates 11-E,Z as well as with methylthiopyridines 19alpha-gamma. In the case of both 11 and 19 oxidative addition to Pd(0) was greatly facilitated in the presence of BF(3) x Et(2)O. The importance of Lewis acid activation to Pd(0) oxidative addition in such substrates appears to be a general phenomenon not previously recognized.     

Copper(II)-catalysed addition of O-H bonds to norbornene

Cu(OTf)2 is an inexpensive, air- and moisture-stable catalyst for the O-H addition of aliphatic and aromatic acids and alcohols to norbornene.     

Manganese(III)-mediated oxidative radical addition of malonates to 2-cyanoindoles

2-Cyanoindole and N-methyl-2-cyanoindole undergo manganese(III)-mediated radical addition with activated methylene and methine compounds. Products of the methylene addition underwent additional oxidation during the course of the reaction to furnish the corresponding acetoxy compounds. Several structures were confirmed by X-ray crystallography.     

Photochemisch induzierte oxidative addition von dichlormethan an einen platin(0)-komplex

Photoinduced oxidative addition of the solvent CH₂Cl₂ to (Ph₃P)₂Pt(C₂H₄) with formation of cis/trans-(Ph₃P)₂PtCl(CH₂Cl) and cis-Cl₂Pt(PPh₃)₂ is observed over several days at ambient temperature. In the presence of the double ylide (R₃Si)₂NP( NSiR₃)₂, R  CH₃, this reaction occurs even in the dark. Sunlight or irradiation with a 500 W lamp increases, whereas addition duroquinone decreases, the rate of this addition reaction. This indicates that, at least in the presence of natural light (wavelength > 290 nm), the formation of free radicals is involved.     

The first definitive example of oxidative addition of acyclic vinyl selenide to M(0) complex

The principle of C-S bond activation of acyclic vinlyl sulfide by platinum(0)-complex was applied to the C-Se bond fission of vinyl selenide. The substrate possessing Ph and ArSe (Ar = C₆H₄Cl-p) substituents at the β-carbon successfully reacted with Pt(0)-complex at 25 °C to produce the vinyl platinum in good yield and its structure was unambiguously determined by X-ray crystallographic analysis. When (Z)-(Me₃Si)(ArSe)C(H)(SeAr) was employed as a reaction substrate, following β-Se elimination took place to liberate Me₃SiCCH with the production of [trans-Pt(SeAr)₂(PPh₃)₂]. The oxidative addition of C-Se bond of (E)-(Ph)(H)CC(H)(SeAr) to Pt(0) was also confirmed at 25 °C, while no C-S bond-breaking occurred when the corresponding vinyl sulfide was exposed to the same reaction conditions, demonstrating that the cleavage of C-Se bond was more facile than that of C-S bond.